Method of treating objects of oxidic,ceramic,polycrystalline materials



July 29, 1969 Filed Oct. 15, 1965 G. H. JONKER 3,458,455

METHOD OF TREATING OBJECTS OF OXIDIC CERAMIC, POLYCRYSTALLINE MATERIALS Sheets-Sheet 1 10 I E E E 510 K 0 1.0 HG TPC) 200 INVENTOR. GERARD HJONKER July 29, 1969 JONKER 3,458,455 METHOD OF TREATING OBJECTS OF OXIDIC CERAMIC, POLYCRYSTALLINE MATERIALS Filed TH 13, 1965 3 $h88t$$ht 2 2..-... MM- V v A INVENTOR. GERARD HJONKER July 29, 1969 5 H. JONKER 3,458,455

METHOD OF TRDIATING OBJECTS OF OXIDIC CERAMIC, POLYCRYSTALLINE MATERIALS Filed on. 13, 1965 3 Sheets-Sheet 3 10 E 6 gm .C 9 as 10 10 r 0 1.0 so 120 160 F|G 3 T (c) INVENTOR. GERARD HJONKER United States Patent 3,458,455 METHOD OF TREATING OBJECTS OF OXIDIC, CERAMIC, POLYCRYSTALLINE MATERIALS Gerard Heinrich Jonker, Emmasingel, Eindhoven, N etherlands, assignor, by mesne assignments, to US. Philips Corporation, New York, N.Y., a corporation of Delaware Filed Oct. 13, 1965, Ser. No. 495,611 Claims priority, application Netherlands, Oct. 17, 1964, 6412123 Int. Cl. 1101b 1/06 US. Cl. 252-521 13 Claims The invention relates to a method of treating objects consisting wholly or partly of oxidic, ceramic, polycrystalline, semiconducting, ferro-electric materials and to objects thus treated. These materials are to denote here ferro-electric materials which have been rendered semiconducting by applying the principle of controlled valency, and, as the case may be, by additional reduction. Such materials are described, for example, in United States Patent 2,841,508, and British Patent 714,965.

It is known that the electrical resistance of oxidic, ceramic, polycrystalline, semiconductive materials formed by the application of the principle of controlled valency and heated in an oxygen-containing medium may increase considerably at an increase in temperature up to or just in excess of the ferro-electric Curie temperature. Examples of such materials are alkaline earth metatitanates, (Perowskite structure), in which part of the alkaline earth ion, for example Be ions, is replaced by La ions, or a part of the Ti ions is replaced by Nb+ ions.

It is supposed that the electrical resistance of these polycrystalline materials above the ferro-electric Curie temperature is chiefly determined by barrier layers which are found at the boundaries of the separate crystals (grain boundaries) of the polycrystalline material. These barrier layers will be termed hereinafter grain boundary barrier layers.

It is furthermore described that for sintered bodies consisting of doped barium titanate of the composition Ba Y TiO above the ferro-electric Curie temperature the curve indicating the relationship between the electrical resistance and the temperature has a steeper slope according as the partial oxygen pressure of an argonoxygen mixture in which the bodies are heated is higher. After heating in pure argon bodies were obtained, the said curve of which was substantially fiat; the electrical resistance of such a body does substantially not increase when the temperature of the body rises.

I have now found that the temperature dependence of the electrical resistance at and in excess of the ferro-electric Curie temperature of bodies consisting wholly or partly of oxidic, ceramic, polycrystalline, semiconductive ferro-electric materials, which have been rendered semiconductive by applying the principle of controlled valency, can be acted upon to a considerable extent.

It has been found that by an increase in temperature up to and in excess of the ferro-electric Curie temperature the electrical resistance of such bodies treated in accordance with the invention increases considerably more strongly than that of bodies manufactured in known manner in air, referred to hereinafter as untreatedlbodies or that the absolute increase in electrical resistance in a given temperature region is considerably greater than that of untreated bodies; both effects frequently appear in a body treated according to the invention. In accordance with the invention, bodies with a strong increase in resistance may be obtained which exhibit a substantially constant temperature coefiicient of the electrical resistance in a wide temperature range.

As a result objects manufactured in accordance with the invention may be employed advantageously for resistors having a positive temperature coefiicient, which resistors may be used in many places for example for current limitation or current stabilisation, in thermoregulators, voltage stabilisers for safeguarding electric motors and for temperature and radiation measurements,

I assume that grain boundary barrier layers are formed in the method according to the invention in the bodies of said polycrystalline materials or that these layers are acted upon. This method does not intendto aifect the semiconductivity of the grain masses.

The method according to the invention is characterized in that an object consisting wholly or partly of oxidic, ceramic, polycrystalline, semiconductive, ferro-electric material, which has been rendered semiconductive by applying the principle of controlled valency, and the electrical resistance of which exhibits a given extent of temperature dependence subsequent to heating of the object in an oxygen-containing atmosphere up to and in excess of the ferro-electric Curie temperature, is subjected at a higher temperature to the action of one or more halogens of the group consisting of fluorine, chlorine and bromine.

The method according to the invention is particularly important for polycrystalline bodies (objects) consisting wholly or partly of oxidic, ceramic, ferro-electric materials, rendered semiconductive by applying the principle of controlled valency, having Perowskite structure such as titanates, zirconates, stannates, niobates of alkaline earth metals of the group of barium, strontium and calcium and of lead and solid solutions of two or more of these compounds. These materials contain suitable quantities of at least one element which renders them semiconductive in accordance with the principle of controlled valency. Examples of such an element are bismuth, antimony, niobium, tantalum and rare earth metals such as lanthanum.

Suitable oxidic, ceramic, ferro-electric materials with Perowskite structure for the objects to be treated in accordance with the invention are for example alkaline earth titanates and solid solutions thereof or of one or more of them with lead titanate, furthermore solid solutions of an alkaline earth titanate and an alkaline earth zirconate or stannate with Perowskite structure or of the two last-mentioned compounds.

Particularly suitable oxidic, ceramic, ferroelectric materials with Perowskite structure for objects to be treated in accordance with the invention are barium titanate, solid solutions of barium titanate and strontium titanate, and/or calcium titanate and solid solutions of barium titanate and barium stannate and of barium titanate and lead titanate. Especially suitable materials are those mentioned in the preceding sentence in which lanthanum, antimony or niobium are used for obtaining the semiconductivity.

The method according to the invention in which the objects to be treated at higher temperatures are subjected to the action of one or more of the halogens of fluorine, chlorine and bromine, may be carried into elfect in different ways.

The result of the treatment according to the invention does not only depend upon the chemical composition of the objects, but also upon a few further factors. One of these factors is the porosity of the body obtained by molding and sintering. The porosity must not be great, since in this case the halogen used would have an excessive effect on the outer layers of the crystals of the polycrystalline body. The body subjected to a treatment according to the invention must neither be sintered to excessive compactness, since then the grain boundaries are inadequately accessible for the halogen vapour. In practice it can be assessed by means of a few systematically performed sintering tests for each individual case how these tests have to be carried out in order to obtain the desired result from the treatment according to the invention.

Moreover, the grain size of the polycrystalline material is a factor affecting the result of the method according to the invention. Use is preferably made of material having a grain size of about 5 to 20p diameter; satisfactory results were obtained with a material having a grain size between 2 and 1001/. diameter.

The treatment of the polycrystalline bodies according to the invention is performed at an increased temperature, for example at a temperature lying between 750 C. and 1200 C., preferably at a temperature lying between 850 C. and 950 C.

Among the halogens particularly chlorine may beemployed. A treatment according to the invention with chlorine can provide a very strong temperature dependence of the electrical resistance in certain cases, while then or in other cases the absolute rise of the electrical resistance with an increase in temperature is surprisingly high. In given cases a treatment according to the invention with chlorine provides bodies having a strong increase in resistance and having a substantially constant temperature coeflicient of the electrical resistance in a long temperature range. Also the use of fluorine or bromine may yield satisfactory results.

The method according to the invention may also be carried out with halogen compounds. The conditions are then chosen preferably so that in situ (at the temperature of the treatment) halogen is developed.

The method according to the invention may be carried out by introducing a halogen vapour or a mixture of a halogen vapour and an inert gas, for example nitrogen or of a halogen vapour and oxygen into a furnace containing the body to be treated. The halogen vapour, mixed or not mixed with said further gases, may be passed over the body to be treated. The treatment may also be carried out in a closed space.

In a further embodiment of the method according to the invention the vapour of a halogen compound or of such a compound together with oxygen may be used instead of the halogen vapour. In this case the halogen compound will usually dissociate or burn at the temperature of the treatment, while halogen vapour is developed. A particularly suitable halogen compound, is an organic compound for example carbon tetrachloride or a fluoric hydrocarbon. As an alternative, voltatile inorganic halogen compounds may be used, for example SiF together with oxygen. The volatile halide may be formed locally.

It has been found that particularly with the use of a mixture of a halogen or volatile halogen compound and oxygen satisfactory results are obtained.

The pressure of the halogen vapour to which the bodies to be treated in accordance with the invention are exposed, may be different. This pressure is preferably chosen to exceed about 0.05 atmosphere. The tests to 4 be described hereinafter are carried out'chiefiy with halogen vapour of pressures varying between 0.01 and 5 atmospheres. It was found that with a pressure of 0.1 atmosphere satisfactory to very satisfactory results were obtained in most cases. For this reason said pressure is preferred.

It was furthermore found that the result of a treatment according to the invention with a mixture of a halogen or a volatile halogen compound and oxygen also depends upon the oxygen pressure. This applies particularly to volatile inorganic fluorine compounds, such as SiF and TiF Favourable results are obtained particularly with the use of higher oxygen pressure, i.e. pressures between 0.8 and 5 atmospheres and higher.

The method according to the invention and the results obtained will be described more fully with reference to the following examples.

In the tests made pellets of oxidic, ceramic, polycrystalline, semiconductive, ferro-electric material were treated together with a halogen or a volatile halogen compound. in conjunction or not in conjunction with oxygen. In some cases the volatile halide was developed locally.

In the first series of tests, pellets of polycrystalline barium titanate containing an excess quantity of 2 mol percent of titanium dioxid and 0.2 mol percent of Nb O were used. First the powder was produced by mixing and grinding BaCo Ti0 and Nb O in the desired ratio. This powder was heated at l000 C. for 15 hours. The resultant product was ground in the wet state and molded into pellets, which were heated at 1350 C. for three hours in air.

A pellet to be tested was provided with ohmic elec trodes, after which the electrical resistance was measured at different temperatures. The electrodes were mounted after removing surface layers (by abrading) from those places where the electrodes had to be connected. The electrode material was an indium mercury alloy; in this way contact resistances are avoided.

In the following tables there is indicated in order of succession the number of the test, the gas atmosphere to which the pellets were exposed, the temperature thereof and the duration of heating, the minimum value of the electric resistivity in ohm. cm., indicated by R (this minimum value lies usually 20 to 60 below the ferroelectric Curie temperature), the temperature in degrees centigrade at which the maximum value of the electric resistivity was measured, indicated by T the ratio between the maximum and minimum values of the resistivity, indicated by R /R the maximum'value of the temperature coefiicient of the electric resistivity indicated by TC in percent per degree centigrade.

The definition of the temperature coefficients (TC) of electric resistance is, given by:

(T temperature in degrees centigrade).

Since in the present case the values of the temperature coefficients are very high, 30 to 120% per degree centigrade, this means that the useof' the electric resistance at a temperature of 1 C. higher, the coefficient is much higher than 30 to 120% since this increase is:

Br n

(e is the base number of the natural logarithm system). For a TC value-of per degree centigrade e =2.78, which means that per degree centigrade the electric resistance increases in this case by a factor 2.78.

The tables also indicate the said magnitudes of discs not subjected to a treatment according to the invention. These cases are indicated in the tables by untreated." During the manufacture as described above these pellets were heated at 1350 C. in air.

6 percent of TiO;,, in excess and 0.15 mol percent of La O The discs to be treated by the method according to the invention had been made by the same process as those the results of which are indicated in Table I. The magnitudes of Table 11 have the same meaning as those of Table I.

TABLE I.MATE RIAL, BARIUMTITANATE WITH AN EXCESS QUANTITY OF 2 MOL. PERCENT OF T102 AND 0.2 MOL. PERCENT OF NbzOr G85 p re, etc. Rmin. Tm. Rama/Rm... Tom.

Test No.

1 Untreated 165 8,000 30 2 Chlorine:0.1 atm:900 0.:2 hours... 100 190 50,000 100 3 Bromine:0.25 atm:900 0.:2 hours 105 165 70,000 90 4 Fluorine :0.05 atm:900 C. (with nitrogen as an inert gas) 65 165 16, 000 CC14:0.1 atm. 2920 0.:2 hours (0014 aera 180 165 42,000 115 155 165 25, 000 100 170 175 000 60 165 25, 000 310 142 60, 000 375 160 40,000 75 185 110, 000 40 345 185 100, 000

FIG. 1 shows graphically the temperature dependence of the electrical resistivity of pellets in the tests 1, 3, 7.

For the pellets of the two further tests of Table I and of the tests to be described hereinafter the curves were made which indicate the temperature dependence of the electrical resistivity. The curves shown in FIG. 1 serve by way of example.

Tabl II indicates a method of treatment and the results obtained with discs of Ba Sr TiO having 1 mol The same applies to Table III for a barium strontium titanate of the composition Ba 7oSI'030TiO with 2 mol percent of TiO; in excess quantity and 0.15 mol percent of Sb O to Table IV for a barium strontium titanate of the composition Ba Sr TiO with an excess quantity of 2 mol percent of Ti0 and 0.15 mol percent of Sb O and to Table V for a barium calcium strontium titanate with an excess quantity of 2 mol percent of TiO and 0.15 mol percent of 812 0 TABLE IL-MATERIAL: Bao.aoSro.20TiOa WITH AN EXCESS QUANTITY OF 1 MOL. PERCENT OF T102 AND 0.15 MOL. PERCENT OF LazOa Gas atmosphere, etc. min. Tmax. RmnX-/ mln- TCmx.

Test N 0.:

13 Untreated 45 160 62,000 16 14 Volatile F-compound formed locally from 100 160 50,000 20 hours.

TABLE III-MATE RIAL: BiioJflShmoTiOgWITH AN EXCESS QUANTITY OF 2MOL. PERCENT OF T102 AND 0.15 MOL. PERCENT OF SbzOa Gas atmosphere, etc. min. Tm. max-l min. ma:-

Test No 15 Untreated 40 800 8 16 C2HC13:0.3 atm.:950 0.:2 hours (open tube 02 flow) 140 6,000 14 17 .1 B!2:0.25 atm.:900 0.:2 hours (closed tube) 200 150 4, 500 10 18 OF2:0.O5 atm.:900 0.12 hours (closed tube) 300 l60 20, 000 15 TAB LE IV.MATE RIAL:B% 0:mSru.soTiO3 WITH AN EXCE SS QUANTITY OF 2 MOL. PE RCENT OF T102 AND 0.15 MOL. PERCENT 013 513203 Gas atmosphere, etc. min. Tmax. max-l min. Tcmnx- TestNo.: A

19 Untreated 40 140 1,000 9 20. 0F2:0.05 atm. :900 C. :2 hours (closed tube) 300 ,000 15 21 Volatile F-compound formed locally from 300 100 30,000 18 (closed tube).

TABLE V.-MATERIAL: B8n.soCau.mSro.ao WITH AN EXCESS QUANTITY OF 2MOL. PERCENT OF T102 AND 0.15 MOL. PERCENT OF Sb2Os Gas atmosphere, etc. Ruin. Tmux. Roman/ umin- OM.

Test No.:

22 Untreated 90 150 500 6 23. 012:0.1 atm.:950 C.:1 hour (closed tube) 140 10, 000 12 I 24 N F3:0.1 atm.:850 0.:2 hours (closed tube).. 400 150 30, 000 15 Table VI indicates the results of tests carried out on resistivity in a wide temperature range. This may be impellets of barium calcium titanate and barium calcium portant for temperature measurements. Said property is strontium titanate with an increasing content of strontium. usually found in objects treated by the method accord- (Material: B2011 Cam $10.2 TiOa with 0.9 mol. percent of TiOz and 0.15 mol. percent of 81020 Untreated 70 160 1,500 9 0130.1 atm.:950 0.:1 110111-.. 140 155 8,000 13 (Material: 1380.6 Ca SIM Ti03 with 0.9 mol. percent of T10; and 0.15 mol. percent of Sbz05) Untreated 70 160 500 6 012:0.1 atmzz950 0.:1 hour 150 140 8, 500 12 FIG. 2 shows graphically the temperature dependence ing to the invention and consisting of materials having of the electrical resistivity (p) of pellets of the tests 25 a comparatively low ferro-electric Curie temperature, for

to 32 example barium strontium titanate, barium calcium stron- For pellets of barium strontium titanate with Si tium titanate, barium stannate titanate and barium ziras a sintcring agent the results are indicated in Table VII. conate titanate.

TABLE VII.-MATE RIAL: Bantu Srozo T103 %%OOF WEIGHT SiOz AND 0.2 MOL. PERCENT Gas atmosphere, etc. Rmln. Tm. mnL/Rmin. TCm-x.

Test No.:

33 Untreated 80 200 11,000 9 34 Volatile F-cornpound locally formed from 85 200 120,000 11 KF+SiO +MuOz (nan) 900 C.:2hours.

For pellets of a barium titanate stannate doped with High values of the temperature coeflicient of the elecniobium and lanthanum respectively the test results are trical resistance are particularly found in objects treated indicated in Table VIII.

TABLE VHI.MATERIAL: Ba(Snn.10Tiu.vo) 0; WITH 0.2 MOL. PERCENT OF NbzOs Gas atmosphere, etc. minmlt. mlL/ min. TCm.

Test No.:

35 Untreated 100 170 3,000 36 Volatile F-compound formed locally from 250 125 54,000

KF+SiO2+M11Oz (1:221) 900 0.:2hours. 37 Untreated 55 170 10,000 13 38 Volatile F-compound formed locally from 250 125 70,000 20 KF+SlOz+MnO2 (1:2:1) 900 C.:2hou.rs.

1 Material: like and 36 but with 0.2 mol. percent of LazOa instead 01 Nbz05.

FIG. 3 shows graphically the temperature dependence in accordance with the invention consisting of materials of the electrical resistivity of pellets of test 36 as an having a comparatively high ferro-electric Curie temperexample of an object according to the invention, which 60 ature, for example barium titanate and barium lead titahas a constant temperature coeflicient of the electrical nate. See TablesIXand XI.

TABLE IX.MATE RIALI BMMPbMsTiO; WITIEIQ (lgdIOL PERCENT T10 AND 0.2 MOL PERCENT GBIS atmosphere. etc. Bruin. Tmnx- RmcL/ min. Tom.

Test No.:

Q0 Untreated 27 220 5, 500 45 40... Volatile F-compound formed locally from C8Fz+ 75 240 100,000 38 S102 (2:1) in Ozatm; 900 0.: (closed tube).

41. NF3:0.1 atm.: 850 0.: 2 hours (closed tube) 30 240 17, 000 40 42 Untreated 220 1,500 40 43 01 10.05 atm.: 900 6.: 2 hOUIS 230 10, 000 63 44 Brz:0.26 atm.: 900 0.: 2 hours.-. 60 210 6, 000 45 1 Fz:0.05 atm.: 900 C. 1 hour 82 200 5, 30 46 1 Volatile F-compound formed locally from KF+ 240 110, 000 85 SiOa+MnOi (1:2:1) 900 0.12 hours (closed tube).

1 Material like 39-41 but with 0.2 mol. percent LazOa instead Nb205.

9 Table X indicates results obtained with pellets of a barium calcium titanate in tests in which on the one hand the influence of the heating time and on the other hand the influence of the temperature are determined.

10 body of the material at an elevated temperature between 750 C. and 1200 C. in an atmosphere containing at least one halogen selected from the group consisting of fluorine, chlorine and bromine.

TABLE X.MATERIAL: BamCamTiO WITH 2 MOL. PERCENT OF TiOz AND 0.15 MOL. PERCENT OF SbzOs Gas atmosphere, etc. Rmin. Tmnx. Ruin/Rm. TOM.

For pellets of barium titanate the influence of the temperature in the treatment according to the invention was assessed and the results are indicated in Table XI.

4. A method as set forth in claim 3 wherein the ferroelectric material is selected from the group consisting of barium titanate, and solid solutions of barium titanate TABLE XI.-M.ATERIAL:MBaTiO3 WITH 2 MOL. PERCENT OF TiOz AND 0.2

0L. PERCENT OF Nb205 From these tests it has been found that differences in the temperature of the treatment according to the invention have a greater influence on the results than ditferences in the pressure of the chlorine gas.

At higher temperature the outer layers of the objects 35 are often chemically attacked. Optimum results (see also Table XI) are obtained at a temperature lying between 850 C. and 950 C. in the method according to the invention.

What is claimed is:

1. A method of improving the properties of a controlledvalence oxide semiconductor constituted at least in part of a ceramic, polycrystalline, ferroelectric material exhibiting a resistivity in the semiconductive range and a positive temperature coefiicient of electrical resitsance up to and in excess of its ferroelectric Curie temperature, comprising the step of heating the said semiconductive material at an elevated temperature lying between 750 C. and 1200" C. while exposed to at least one halogen selected from the group consisting of fluorine, chlorine and bromine.

2. A method as set forth in claim 1 wherein the material is a sintered body of a material having a Perowskite structure.

3. A method of improving the properties of a controlledvalence oxide semiconductor constituted mainly of a sintered, ceramic, polycrystalline, ferroelectric material having a Perowskite structure and selected from the group consisting of titanates, zirconates, stannates, niobates, and solid solutions thereof, of a metal selected from the group consisting of barium, strontium, calcium and lead and exhibiting a resistivity in the semiconductive range and a positive temperature coeflicient of electrical resistance up to and in excess of its ferroelectric Curie temperature, comprising the step of heating a compressed, powdered with at least one member selected from the group consisting of strontium titanate, calcium titanate, barium stannate, and lead titanate.

5. A method as set forth in claim 4 wherein the controlled valence is produced by the addition of lanthanum, antimony or niobium.

6. A method as set forth in claim 3 wherein the halogen is chlorine.

7. A method as set forth in claim 3 wherein the halogen is derived from a halogen compound.

8. A method as set forth in claim 3 wherein the atmosphere contains an inert gas.

9. A method as set forth in claim 3 wherein the atmosphere contains oxygen.

10. A method as set forth in claim 9 wherein the oxygen pressure is at least 0.1 atmosphere at the elevated temperature.

11. A method as set forth in claim 10 wherein the oxygen pressure is 0.8-5 atmospheres at the elevated temperature.

12. A method as set forth in claim 3 wherein the elevated temperature lies between 850 C. and 950 C.

13. A method as set forth in claim 3 wherein the powdered material has a grain size between 2 and microns.

References Cited UNITED STATES PATENTS 3,111,414 11/1963 Buessen et a1. l06-39 W. L. JARVIS, Primary Examiner US. Cl. X.R. 

1. A METHOD OF IMPROVING THE PROPERTIES OF A CONTROLLEDVALENCE OXIDE SEMICONDUCTOR CONSTITUTED AT LEAST IN PART OF A CERAMIC, POLYCRYSTALLINE, FERROELECTRIC MATERIAL EXHIBIING A RESISTIVITY IN THE SEMICONDUCTIVE RANGE AND A POSITIVE TEMPERATURE COEFFICIENT OF ELECTRICAL RESISTANCE UP TO AND IN EXCESS OF ITS FERROELCTRIC CURIE TEMPERATURE, COMPRISING THE STEP OF HEATING THE SAID SEMICONDUCTIVE MATERIAL AT AN ELEVATED TEMPERATURE LYING BETWEEN 750*C. AND 1200*C. WHILE EXPOSED TO AT LEAST ONE HALOGEN SELECTED FROM THE GROUP CONSISTING OF FLUORINE, CHLORINE AND BROMINE. 